Catalase oxidation mechanisms

FIGURE 6.11 Typical current—time responses of Fe-SOD/MPA-modified Au electrode toward 02 in 25 mM phosphate buffer (02-saturated, pH 7.5) containing 0.002 unit of XOD upon the addition of 50 nM xanthine at +300 (a) and —lOOmV (b). The arrows represent the addition of 10 j,M of Cu, Zn-SOD (a) and 580 units of catalase and 10 pM of Cu, Zn-SOD to the solution (b). The solution was stirred with a magnetic stirrer at 200rpm. Inset mechanism for the amperometric response of SODs/MPA-modified Au electrodes to 02, based on enzymatic catalytic oxidation (a) and reduction (b) of 02 (M metal ions of SODs). (Reprinted from [138], with permission from the American Chemical Society.)... 

The mechanism of H02 formation from peroxyl radicals of primary and secondary amines is clear (see the kinetic scheme). The problem of H02 formation in oxidized tertiary amines is not yet solved. The analysis of peroxides formed during amine oxidation using catalase, Ti(TV) and by water extraction gave controversial results [17], The formed hydroperoxide appeared to be labile and is hydrolyzed with H202 formation. The analysis of hydroperoxides formed in co-oxidation of cumene and 2-propaneamine, 7V-bis(ethyl methyl) showed the formation of two peroxides, namely H202 and (Me2CH)2NC(OOH)Me2 [16]. There is no doubt that the two peroxyl radicals are acting H02 and a-aminoalkylperoxyl. The difficulty is to find experimentally the real proportion between them in oxidized amine and to clarify the way of hydroperoxyl radical formation. 

It follows from the above that MPO may catalyze the formation of chlorinated products in media containing chloride ions. Recently, Hazen et al. [172] have shown that the same enzyme catalyzes lipid peroxidation and protein nitration in media containing physiologically relevant levels of nitrite ions. It was found that the interaction of activated monocytes with LDL in the presence of nitrite ions resulted in the nitration of apolipoprotein B-100 tyrosine residues and the generation of lipid peroxidation products 9-hydroxy-10,12-octadecadienoate and 9-hydroxy-10,12-octadecadienoic acid. In this case there might be two mechanisms of MPO catalytic activity. At low rates of nitric oxide flux, the process was inhibited by catalase and MPO inhibitors but not SOD, suggesting the MPO initiation. 

General descriptors may be related to the metabolism responses in the biofilm. Biofilm algae have several mechanisms to counterbalance the damage caused by the toxicants. Environmental stress produces oxidative damage in the cells, which can be tracked down by means of the analysis of many enzymes (superoxide dismutase, catalase, peroxidase, etc.) that function as effective quenchers of reactive oxygen species (ROS). 

On the other hand, several ROS are highly cytotoxic. Consequently, eukaryotic cells have developed an elaborate arsenal of antioxidant mechanisms to neutrahze their deleterious effects (enzymes such as superoxide dismutases, catalases, glutathione peroxidases, thioredoxin inhibitors of free-radical chain reaction such as tocopherol, carotenoids, ascorbic acid chelating proteins such as lactoferrin and transferrin). It can be postulated that ROS may induce an oxidative stress leading to cell death when the level of intracellular ROS exceeds an undefined threshold. Indeed, numerous observations have shown that ROS are mediators of cell death, particularly apoptosis (Maziere et al., 2000 Girotti, 1998 Kinscherf et al., 1998 Suzuki et al., 1997 Buttke and Sanstrom, 1994 Albina et al., 1993). 

The early work on catalase expression was carried out largely in E. coli and revealed two main response mechanisms. One or the other or both responses have been identified in most other bacteria expressing a catalase. The expected and most obvious response is to oxidative stress. Addition of hydrogen peroxide directly or of ascorbate, which... 

Fig. 14. A mechanism to explain heme modification in the P. vitcde catalase and possibly E. coli HPII. For simplicity, the phenyl ring of T3rr415 is not shown, and only ring III of the heme and the heme iron are shown. Compound I is an oxyferryl species formed, along with water, in the reaction of one H2O2 with the heme. The iron is in a formal Fe oxidation state, but one oxidation equivalent is delocalized on the heme to create the 0x0-Fe" -heme cation, shown as the starting species, compound I. A water on the proximal side of the heme is added to the heme cation species of compound 1 shown in A to generate a radical ion in B. The electron flow toward the oxo-iron would generate the cation shown in (C), leading to the spirolactone product shown in D. In E, an alternate mechanism for the His-Tyr bond formation in HPII is presented that could occur independently of the heme modification reaction. Reprinted with permission of Cambridge University Press from Bravo et al. (93).

Catalases have proven to be a treasure trove of unusual modifications. The first noted modification was the oxidation of Met53 of PMC to a methionine sulfone (77). Met53 is situated in the distal side active site adjacent to the essential His54 in a location where oxidation by a molecule of peroxide would not be unexpected. Among the catalases whose structures have been solved, PMC is unique in having the sulfone because valine is the more common replacement in other catalases. The sulfone does not seem to have a role in the catalytic mechanism and is clearly generated as a posttranslational modification. A small number of catalases from other sources, principally bacteria, have Met in the same location as PMC, and it is a reasonable prediction that the same oxidation occurs in those enzymes as well, although this has not been demonstrated. 

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